This invention relates to the refining of bismuth and, more particularly, to an improved process for producing substantially pure bismuth from the debismuthizing dross obtained in the lead refining process.
The smelting of lead ores provides a bullion containing a number of "impurities" which are removed in various well-known refining steps to produce commercially pure lead. In general, the bullion is first decopperized, followed by a softening operation to remove elements such as tin, antimony and arsenic. The softened lead is then desilverized resulting in a desilverized bullion containing substantially lead and bismuth.
It is known in the prior art to remove the bismuth from the lead by treating a molten bath of lead-containing bismuth with appropriate alkaline earth or alkali metal as debismuthizing reagents. The bismuth forms an alkaline bismuthide (e.g., Ca Mg.sub.2 Bi.sub.2) with the debismuthizing reagents and the alkaline bismuthide forms a dross on the molten lead bath surface, the dross being referred to in the art as alkaline bismuthide dross or debismuthizing dross. Calcium and magnesium are presently utilized as debismuthizing reagents for removing the bismuth from the lead. Sodium and potassium have also been utilized as the debismuthizing reagent. The dross is then separated from the lead, for instance by skimming. The debismuthizing process is described in U.S. Pat. No. 5,041,160, which patent is hereby incorporated by reference.
Unfortunately, the debismuthized dross contains mostly lead and both the lead and bismuth must be recovered from the dross for environmental and economic considerations. A typical dross contains, by weight, about 3 to 6% bismuth, 0.4 to 1.5% calcium and 1 to 2% magnesium, with the balance being essentially lead.
The separation and recovery of the bismuth from the lead is basically achieved by a series of steps which remove lead as a slag and increases the bismuth content of the remaining dross material until substantially pure bismuth is obtained.
One procedure to recover bismuth from debismuthizing dross is shown in U.S. Pat. No. 4,039,322 whereby solid alkaline debismuthide is physically separated from molten lead by vacuum filtration. This process increases the bismuthide concentration of the dross and reduces the amount of lead that must be removed in the remaining refining steps.
A commercial process for treating dross is to first remove the calcium and magnesium (usually termed upgraded dross) and then to halogenate the upgraded dross forming a PbCl2 slag. This is continued until substantially pure bismuth is obtained. The PbCl2 slag is generally recycled to treat the dross before halogenation to remove calcium and magnesium, which slag is used to treat the debismuthized lead obtained in the drossing step.
A process for refining debismuthizing dross is shown in U.S. Pat. No. 2,955,931 (Flores et al.), which patent is hereby incorporated by reference. In general, the basic process upgrades and smelts the dross according to known procedures to yield a product containing about 25-40% by weight bismuth, with the calcium and magnesium being removed in the form of slags. The product is then cupelled to increase the dross to the range of about 50-65% by weight bismuth. Above this level the dross is halogenated using chlorine gas to remove the remaining lead as lead chloride and leaving a relatively pure bismuth product.
In the improved process of Flores et al. the halogenation rate is increased by performing the halogenation in a bath wherein the bismuth content is maintained over 85%. This is accomplished by using a pool of high bismuth content lead alloy and adding the lower level bismuth dross (50-65%) in an amount and rate to maintain the bismuth content of the total bath above 85% during the halogenation reaction.
While the above processes for refining bismuth produce a high purity bismuth product, there is still a need for increasing the rate of recovery and/or yield of the bismuth product from the original feed material by accelerating the separation reaction and/or minimizing the amount of bismuth carried over in the slagging steps.
Accordingly, an object of the present invention is to provide an improved bismuth refining process which provides increased bismuth recoveries and rates of recovery.
A further object is to provide an improved bismuth recovery process in the lead refining process wherein debismuthized dross is treated to recover bismuth therefrom.
Other objects and advantages will be apparent from the following description.